1. Field of the Invention
The present invention relates to a concentration detector. More particularly, the invention relates to a concentration detector for detecting the concentration of NOx (nitrogen oxides) in the exhaust gas emitted from an internal combustion engine.
2. Description of the Related Art
Japanese Unexamined Patent Publication (Kokai) No. 8-178896 discloses an air-fuel ratio detector for detecting an oxygen concentration in the exhaust gas emitted from an internal combustion engine and for calculating (detecting) the air-fuel ratio from the oxygen concentration that is detected. The air-fuel ratio detector of the above Japanese Unexamined Patent Publication (Kokai) No. 8-178896 is provided with a pair of electrodes (which, in Japanese Unexamined Patent Publication (Kokai) No. 8-178896, are called pump electrodes and are designated at 12 and 13) for pumping out oxygen from the exhaust gas. When a voltage is applied across the electrodes in the above air-fuel ratio detector, oxygen in the exhaust gas is pumped out. Here, the voltage applied across the electrodes is controlled depending upon the oxygen concentration in the exhaust gas.
In the air-fuel ratio detector disclosed in Japanese Unexamined Patent Publication (Kokai) No. 8-178896 as described above, a voltage corresponding to the oxygen concentration in the exhaust gas is applied across the electrodes. According to Japanese Unexamined Patent Publication (Kokai) No. 8-178896, however, when the internal combustion engine is in a normal operating condition (i.e., in an operating condition excluding an operating condition where no fuel is fed to the internal combustion engine, hereinafter referred to as a “normal engine operating condition”), the voltage applied across the above electrodes is maintained smaller than a predetermined voltage. Therefore, a judgement is rendered that the air-fuel ratio detector is abnormal if the voltage applied across the electrodes is greater than the predetermined value while the engine is normally operating. When no fuel is supplied to the internal combustion engine (hereinafter referred to as a “fuel cut”), however, the oxygen concentration in the exhaust gas increases and becomes nearly equal to the oxygen concentration in the atmosphere. Therefore, the voltage applied across the electrodes becomes greater than the predetermined value even though the air-fuel ratio detector is not abnormal. According to the method of judging an abnormal condition in the air-fuel ratio detector disclosed in the above Japanese Unexamined Patent Publication (Kokai) No. 8-178896, in this case, it is so judged that the air-fuel ratio detector is abnormal. However, it is highly probable that the result of this judgement is incorrect. According to Japanese Unexamined Patent Publication (Kokai) No. 8-178896, therefore, the abnormal condition of the air-fuel ratio detector is not judged when the fuel is being cut.
Namely, according to Japanese Unexamined Patent Publication (Kokai) No. 8-178896, when the air-fuel ratio detector is normally functioning, an abnormal condition in the air-fuel ratio detector is judged based on a prerequisite that the oxygen concentration in the exhaust gas is smaller than a predetermined concentration during the normal operation of the engine, but the abnormal condition in the air-fuel ratio detector is not judged when the fuel is cut because the oxygen concentration in the exhaust gas becomes greater than a predetermined concentration. When the fuel is cut as described above, the oxygen concentration in the exhaust gas increases rapidly. In order for the air-fuel ratio detector to work normally, therefore, the air-fuel ratio detector must be furnished with a particular control operation.
There has been known a NOx concentration detector for detecting the concentration of NOx (nitrogen oxide) in the exhaust gas. As the NOx concentration detector of this kind, there has been known the one according to which oxygen is removed from the exhaust gas and, then, NOx in the exhaust gas is decomposed to newly form oxygen, and the NOx concentration is estimated from the concentration of oxygen that is newly formed. Even in this NOx concentration detector, the function is affected by the oxygen concentration in the exhaust gas. To maintain a normal function, therefore, the NOx concentration detector must be furnished with some special control means, when the fuel is cut, as described in Japanese Unexamined Patent Publication (Kokai) No. 8-178896.
This applies not only to the NOx concentration detector for detecting the NOx concentration in the exhaust gas but also to general concentration detectors for detecting the concentrations of particular components in the gas and of which the functions are affected by the oxygen concentration in the gas.
It is therefore an object of the present invention to provide a concentration detector for detecting the concentration of a particular component in the gas and maintaining a normal function even when the fuel is being cut.